Liquid scintillation counting is a generally known and widely used technique for the measurement of low energy beta emitting radionuclides in addition to gamma and alpha emitters. The liquid scintillation counter is utilized to make quantitative measurements of the concentration of one or more radionuclides in a solution capable of producing photons resulting from kinetic interactions of nuclear decay products and molecules in the solution. This concentration is directly proportional to the count rate and the counting efficiency of the radionuclide. The counting efficiency is defined as the count rate divided by the disintegration rate.
Beta radiating isotopes constitute the absolutely largest group of radionuclides measured by liquid scintillation counting, and within this group H-3 and C-14 are the two most commonly used isotopes.
In liquid scintillation counting a sample containing radionuclides is dissolved in a mixture with a typical liquid scintillation cocktail, such as an aromatic solvent containing an organic scintillator. The energy of the radio-active decay excites solvent molecules and the energy of excited states of solvent molecules is transferred to excitations of scintillator molecules, and when these excited scintillator molecules return to their ground states, they emit photons. The light is detected by photomultiplier tubes or other photosensitive devices. Most commercially available counters utilize multiple photomultiplier tubes and coincident counting to minimize the background caused by the dark noise of photomultiplier tubes.
The number of photons produced by a radioactive decay is proportional to the energy with which the decay dissipates in the solution. All photons produced within a coincident interval are considered as a pulse. The sum of the amplitudes of the photons of one pulse is known as the pulse height. The range of the pulse heights is a smooth distribution of energies ranging from some minimum energy to some maximum energy. This distribution is known as the pulse height energy distribution spectrum.
The phenomenon of quenching in liquid scintillation counting decreases the number of produced photons. The reduction in the amount of photons shifts the pulse height energy distribution to the left and decreases the rate of counted pulses or the counting efficiency.
Quenching is a consequence of the introduction of some material into the solution, which absorbs primary radiation of radioactive decay or excitations of molecules or photons.
To relate the disintegration rate of a sample to the count rate it is necessary to determine the quench level and correlate it to the counting efficiency. There are different methods to estimate the quench level of a sample. Most of them rely on the effect of quenching on the position of the pulse height energy distribution with respect to a fixed point on the amplitude scale. One can measure the shift in pulse amplitudes of either the spectrum resulting from the dissolved radionuclide or in that of resulting from Compton electrons scattered by gamma rays originating from a radioactive isotope external to the scintillation solution. This isotope is known as the external standard.
A traditional way to measure the pulse amplitude shift is to divide the pulse amplitude scale into two parts: a lower and an upper part and calculate the ratio of the counts in the two parts and use this ratio as a measure of the degree of quenching. This quench indicating parameter is known as the channels' ratio.
Other possible quench indicating parameters are for example the end point of a spectrum or the inflection point of a spectrum. Some counters have two quench indicating parameters of which the other measures the color contents of the sample. The term quench index (and the symbol q) is used henceforth to represent a quench indicating parameter or a vector consisting of more than one quench indicating parameter.
The relationship between a quench index and the counting efficiency of a radionuclide is established by measuring reference samples of known activity and expressing the counting efficiency of these samples as a function of the quench index. The whole procedure is called quench calibration and those functions are called quench calibration functions.
Generally a quench calibration of a quantity means the establishing of a correlation between the values of the quench index and the said quantity. A re-calibration means a calibration in the case, where there already exist a calibration. A quench calibration function is a correlation between the quench index and the calibrated quantity.
To produce a quench calibration, which covers a moderate range of quench levels, several standard samples have to be made and measured. The extrapolation of the quench calibration function out of the range, which the standard samples cover, is quite inaccurate and easily results in systematic errors.
Every radionuclide needs its own standardization and also different liquid solutions and even different kind of quenching materials need their own standardizations. Using of a wrong standard curve in calculating an unknown sample leads to erroneous results. Because in every quench calibration typically from six to ten standard samples are needed at present, an arrangement that decreases the number of standard samples per calibration is valuable.
The shapes of quench calibration functions of one radionuclide resemble each other. So if besides the particular reference samples also the shape of the calibration function of some former calibration can be used, more accurate quench calibrations than before can be made with the same number of standard samples.
At present the only way to utilize any old quench calibration data in making a new quench calibration is to add manually the earlier measured reference sample data to the new reference sample data.